Diode

A semiconductor device with two terminals, typically allowing the flow of current in one direction only.

A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance), it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.

Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators. The fundamental property of a diode is its tendency to conduct electric current in only one direction.

A diode only blocks current in the reverse direction while the reverse voltage is within a limited range otherwise reverse barrier breaks and the voltage at which this breakdown occurs is called reverse breakdown voltage. The diode acts as a valve in the electronic and electrical circuits.

A PN junction is the simplest form of the semiconductor diode which behaves as ideally short circuit when it is in forward biased and behaves as ideally open circuit when it is in the reverse biased.

Beside simple PN junction diodes, there are different types of diodes although the fundamental principles are more or less same. So a particular arrangement of diodes can convert AC to pulsating DC, and hence, it is sometimes also called as a rectifier. The name diode is derived from "di - ode" which means a device having two electrodes.

Symbol of Diode

P-N Junction Diode

PN junction diode is made by doping donor impurity in one portion and acceptor impurity in other portion of silicon or germanium crystal block. These dopings make a PN junction at the middle part of the block beside which one portion becomes p-type (doped with trivalent or acceptor impurity), and another portion becomes n-type (doped with pentavalent or donor impurity). We can also form a PN junction by joining a p-type (intrinsic semiconductor doped with a trivalent impurity) and n-type semiconductor (intrinsic semiconductor doped with a pentavalent impurity) together with a special fabrication technique. Hence, it is a device with two elements, the p-type forms anode, and the n-type forms the cathode. These terminals are brought out to make the external connections.

Forward Bias

In a PN junction diode when the forward voltage is applied i.e. positive terminal of a source is connected to the p-type side, and the negative terminal of the source is connected to the n-type side, the diode is said to be in forward biased condition.

We know that there is a barrier potential across the junction. This barrier potential is directed in the opposite of the forward applied voltage. So a diode can only allow current to flow in the forward direction when forward applied voltage is more than barrier potential of the junction. This voltage is called forward biased voltage. For silicon diode, it is 0.7 volts. For germanium diode, it is 0.3 volts. When forward applied voltage is more than this forward biased voltage, there will be forward current in the diode, and the diode will become short circuited. Hence, there will be no more voltage drop across the diode beyond this forward biased voltage, and forward current is only limited by the external resistance connected in series with the diode. Thus, if forward applied voltage increases from zero, the diode will start conducting only after this voltage reaches just above the barrier potential or forward biased voltage of the junction. The time, taken by this input voltage to reach that value or in other words, the time, taken by this input voltage to overcome the forward biased voltage is called recovery time.

Reverse Bias

In reverse biased p-n junction diode, the positive terminal of the battery is connected to the n-type semiconductor material and the negative terminal of the battery is connected to the p-type semiconductor material.

In reverse biased p-n junction diode, the free electrons begin their journey at the negative terminal whereas holes begin their journey at the positive terminal. Free electrons, which begin their journey at the negative terminal, find large number of holes at the p-type semiconductor and fill them with electrons. The atom, which gains an extra electron, becomes a charged atom or negative ion or motionless charge. These negative ions at p-n junction (p-side) oppose the flow of free electrons from n-side.

On the other hand, holes or positive charges, which begin their journey at the positive terminal, find large of free electrons at the n-type semiconductor and replace the electrons position with holes. The atom, which loses an electron, becomes a charged atom or positive ion. These positive ions at p-n junction (n-side) oppose the flow of positive charge carriers (holes) from p-side.

If the reverse biased voltage applied on the p-n junction diode is further increased, then even more number of free electrons and holes are pulled away from the p-n junction. This increases the width of depletion region. Hence, the width of the depletion region increases with increase in voltage. The wide depletion region of the p-n junction diode completely blocks the majority charge carriers. Hence, majority charge carriers cannot carry the electric current.

However, p-n junction diode allows the minority charge carriers. The positive terminal of the battery pushes the holes (minority carriers) towards the p-type semiconductor. In the similar way, negative terminal of the battery pushes the free electrons (minority carriers) towards the n-type semiconductor.

The positive charge carriers (holes) which cross the p-n junction are attracted towards the negative terminal of the battery. On the other hand, the negative charge carriers (free electrons) which cross the p-n junction are attracted towards the positive terminal of the battery. Thus, the minority charge carriers carry the electric current in reverse biased p-n junction diode.

The electric current carried by the minority charge carriers is very small. Hence, minority carrier current is considered as negligible.